A symmetrical isomer of polychlorinated biphenyl, (PCB), 3,3',4,4',5,5'-hexachlorobiphenyl (HCB) was given to pregnant mice on the 6th through 15th days of pregnancy at 0, 0.1, 1.0, 2.0, 4.0, 8.0 or 16.0 mg/kg body weight. At sacrifice on day 18 of gestation, pathologic changes in the maternal and fetal liver were greatest but the microscopic pattern was remarkably different. Changes in the maternal liver were characterized by moderate to marked swelling of hepatocytes, single cell necrosis primarily around the central veins and occasional microabscesses. On the contrary, in the fetal liver there was massive necrosis in the hepatic lobes and formation of a tubule-like pattern of hepatocytes. The tubule-like formation was similar to the hepatotoxic lesions induced by other halogenated polycyclic hydrocarbons in the adult rats. The objective of this study was to find out the toxicity of a pure isomer of PCB (3,3',4,4',5,5'-hexachlorobiphenyl) in the fetus and the pregnant mouse.